1. Field of the Invention
The present invention relates to an optical multiplexing communication method, an optical multiplexing communication system, an optical signal multiplexing device, and an optical multiplexed signal demultiplexing device for optical signals used in a high speed optical transmission system, a wavelength division multiplexing transmission system, an optical signal processing system, etc.
2. Description of the Related Art
A ultra high speed optical transmission system aiming at the realization of an optical communication system with a larger capacity has been under the development in view of the rapid increase of traffics in the data communications. More specifically, the attempt has been made to replace the multiplexing/demultiplexing circuits that have been formed by electronic circuits with all-optical type optical signal processing circuits, so as to eliminate the limitation due to the electronic circuits on the transmission speed.
The basic concept behind such a prior art is that portions such as the optical modulation unit and the photo-detection unit that can be processed by the electronic circuits will be electrically processed at a low speed, while portions that can be processed all-optically will be processed as ultra high speed signals with the operation speed faster than that of the electronic circuit which are realized by the optical time division multiplexing. Based on this concept, the ultra high speed transmission experiments of 400 Gbit/s, 640 Gbit/s, and 1.28 Tbit/s have been reported so far.
FIG. 10 schematically shows a conventional ultra high speed optical communication system using the optical time division multiplexing. As shown in FIG. 10, the optical signals of Fø Hz before the multiplexing that are generated from a light source 51 are all optically time division N demultiplexed at the optical signal multiplexing device (optical multiplexer) 53, electrically modulated by an optical modulator 52, and then N multiplexed again by the optical signal multiplexing device 53, and transmitted through an optical transmission line as the optical multiplexed signals of N×Fø bit/s. The optical multiplexed signals received by an optical multiplexed signal demultiplexing device (optical demultiplexer) 54 are demultiplexed into the optical signals of Fø bit/s in N communication channels, and the demultiplexed optical signals of Fø bit/s are converted into electric signals by photo-detectors 55 provided in correspondence to the respective communication channels and processed electrically.
FIG. 11 shows an exemplary configuration of the conventional optical signal multiplexing device 53. The optical signal multiplexing device 53 of FIG. 11 comprises delay lines 56 capable of realizing the ultra high speed signal pulse sequences of N×Fø bit/s by the N-fold time division multiplexing, and N sets of optical branching couplers 57. Here, the branching ratio of the optical branching coupler 57 is fixed such that the optical signal pulse sequences after the multiplexing have equal amplitudes. FIG. 12 schematically shows a concept of the conventional optical signal multiplexing described above.
The representative optical signal multiplexing device proposed so far is one in which delay lines with delay time differences which are multiples of a prescribed delay time are combined in multiple stages, and the representative optical multiplexed signal demultiplexing device is one in which a nonlinear optical loop mirror (NOLM) of the Sagnac interferometer is combined with the optical Kerr effect.
FIG. 13 schematically shows an exemplary configuration of the conventional optical multiplexed signal demultiplexing device 54. As shown in FIG. 13, the control pulses of Fø bit/s are indispensable in order to demultiplex the ultra high speed signal pulses of N×Fø bit/s into signal components of Fø bit/s. Here, in order to utilize the interference effect that occurs between the optical signals and the control pulse sequences, the control pulse sequences are required to be capable of being synchronized timewise with and having the polarization direction identical to the time division multiplexed input light signals, and to be having the optical intensity sufficiently larger than the optical signals.
Among the prior arts described above, the optical signal multiplexing device has a problem that, when the number of multiplexing N is increased to realize a large capacity, the time interval Tø between the optical signals becomes shorter so that there is a need to narrow down the pulse width T of the optical signals to be multiplexed and use the ultra short optical pulses that are difficult to transmit as the signal lights, and consequently the adaptation to the system is difficult.
On the other hand, the optical multiplexed signal demultiplexing device has a problem that there is a need to provide the high output short pulse light source for the control pulses, and a problem that the synchronization of timings of the optical signals and the control pulses is very difficult so that there is a need for the optical phase lock using an optical phase lock loop 58. In addition, it is often difficult to make the polarization directions identical and there is a need to form the entire system by using medium that can maintain the polarization direction for this reason, but there has been a problem that the system having such a configuration is very expensive.
FIG. 14 shows an exemplary configuration of the conventional optical multiplexer 102. The input optical pulse sequences having the clock frequency of Fø Hz are split into N by an optical intensity splitter 106, and the split optical pulse sequences are encoded by optical intensity modulators 107 having delay lines with delay time differences which are multiples of a prescribed delay time, and multiplexed again by an optical multiplexer 108, and then optical TDM signals of N×Fø bit/s are transmitted. At the optical demultiplexer 103, as shown in FIG. 15, the control pulse sequences having a repetition frequency of Fø Hz are synchronized with the signal lights of N×Fø bit/s by using an optical phase lock loop circuit 109, and the optical TDM signals are demultiplexed into optical signals of Fø bit/s by the nonlinear interaction (optical Kerr effect).
As shown in FIG. 16, the N-fold time division multiplexed ultra high speed signals of N×Fø bit/s are produced. The signal (pulse) interval that was 1/Fø sec. before the multiplexing is multiplied by 1/N by the multiplexing and becomes as short as (1/N)×(1/Fø) sec. Consequently, the synchronization with the control pulses (of the repetition frequency Fø Hz) at the optical demultiplexer 103 becomes increasingly difficult in counter proportion to the level of multiplexing. Also, the signals propagated through the conventional transmission line have the timing displacements due to the environmental change such as that of the temperature so that the synchronization with the control pulses has been even more difficult. In addition, in order for the signal lights and the control lights to interact at high efficiency, it is necessary to make the polarization directions coincide so that there is a need to construct the entire transmission system by a medium that can maintain the polarization direction, but its realization has a problem in terms of the cost.